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Colowyo Coal Company, L.P. <br />Attn.: Mr. Juan Garcia <br />March 10, 2015 <br />Page 4 of 7 <br />SHANNON a ill -SON. INC. <br />Colowyo informed us that the revised sedimentation pond embankments will not classify as <br />jurisdictional according to MSHA criteria (30 CFR 77.216(a)) or the Colorado State Engineers <br />Office (C.R.S. 37-87-105). Because similar or better foundation conditions are anticipated for <br />the revised embankments, it is our opinion that the recommendations in our previous report <br />remain valid. <br />SOUTHERN SIDE OF REVISED SPOIL FILL <br />The southern limit of the revised spoil fill approaches the low wall for the Collom Pit. The <br />stability of the low wall was analyzed by Call and Nicholas, Inc. (CNI, 2005) using Universal <br />Distinct Element Code (UDEC) numerical methods. A FS of 2.5 was calculated by CNI for the <br />low wall using mean rock properties. Colowyo has revised the low wall configuration to include <br />a bench about 280 feet wider at the X -Seam compared to the CNI configuration. The spoil is set <br />back about 165 feet from this revised crest. Based on the CNI stability results we estimated a <br />zone behind the low wall where displacement and yielding influenced the stability calculations. <br />This suggests the spoil slope should have an insignificant influence on low wall stability. <br />We also preformed a stability analysis of the southern spoil slope. Below the southern spoil <br />slope the ground surface slope and bedrock dip is generally towards the north at about 8 to 12 <br />percent in a favorable direction. We understand the base of the spoil will be supported by the <br />existing ground. We performed a stability analyses for this condition with the following <br />parameters: <br />■ Spoil slope height of 250 feet with an average overall slope of 3:1 (H:V), <br />■ Ten percent favorable slope of surface towards the spoil fill, <br />■ Mean spoil shear strength (Figure 6 of previous report). Friction angle of 35 degrees and <br />750 pounds per square foot (pso cohesion, <br />■ Lower bound shear strength for overburden soils (Figure 6 of previous report). Friction <br />angle of 24 degrees and cohesion of 370 psf. <br />• Impenetrable bedrock below soil layer. <br />A FS of 2.1 was calculated with the results of the analysis presented on Figure 5. <br />23-1-01403-002 <br />